Manipulation of electromagnetic radiation, particularly light, has received increasing attention in development of electro-optical signal transmission. One of the principal ways in which electromagnetic radiation can be modified is by polarization. The amount of polarization induced during transmission of electromagnetic radiation is generally determined by the medium through which the signal passes. The medium, in turn, can be characterized in terms of its effect on electromagnetic radiation by the following equation: EQU P=X.sup.(1) E+X.sup.(2) E+X.sup.(3) E.sup.(3) . . .
where:
P is the total polarization induced in the electromagnetic radiation; PA1 E is the local electric field generated by the electromagnetic radiation; and PA1 X.sup.(1), X.sup.(2), and X.sup.(3) are the first, second and third order polarization susceptibilities, respectively, of the electromagnetic wave transmission medium.
Materials which have relatively high second order polarization susceptibilities have been suggested for application in optical rectification and generation of electro-optical effects, such as alteration of the refractive index of the medium and phase alteration of electromagnetic radiation, and generation of parametric effects, such as second harmonic generation (SHG). However, second order polarization susceptibility typically is large enough for such applications only when the molecules of the optical transmission medium is noncentrosymmetric, wherein the molecules are each asymmetrical about their centers. Also, the largest values of second order polarization susceptibility are generally obtained in polymeric systems only by poling optically active molecules, thereby causing the molecular dipoles of the molecules to align.
Optical media which exhibit significant second order polarization effects on light have included, for example, noncentrosymmetric inorganic crystals, such as potassium dihydrogen phosphate and lithium niobate. However, other features of these inorganic crystals, such as difficult, expensive synthesis, and brittleness, have severely limited their range of application. Another attempt has been to incorporate poled (aligned) molecule dipoles, which exhibit second order polarization susceptibility, into an organic matrix which has been formed from an organic monomer. Such materials can be employed as thin films having good optical quality, but historically have had several problems. For example, the poled position of the dipoles is thermodynamically unstable. Therefore, polymer matrixes are usually employed which have a relatively high glass transition temperature, to thereby restrict the reorientation of poled dipoles. However, polymeric materials which have high glass transition temperatures generally cannot be employed in bulk to form articles which are of suitable optical quality for second order polarization. Moreover, poling can diminish with time even at temperatures which are significantly below the glass transition temperature.
Therefore, there is a need for a nonlinear optical composition and a method of forming such a composition which overcome the above-referenced problems.